The present invention generally relates to directional microphones and, more particularly, to a directional microphone having a minimized self noise level in order to achieve improved dynamic range performance.
Directional microphones are widely used in the professional market for various applications such as news gathering, sporting events, outdoor film recording, and outdoor video recording. The use of directional microphones in these types of situations is a necessity where noise is present and there is no practical way to place the microphone in close proximity to the audio source.
Two kinds of directional microphones are in use today. The first type of directional microphone is called a shotgun microphone which is also known as a line plus gradient microphone. Shotgun microphones typically comprise an acoustic tube that by its mechanical structure reduces noises that arrive from directions other than directly in front of the microphone along the axis of the tube. The second type of directional microphone is a parabolic dish that concentrates the acoustic signal from one direction by reflecting away other noise sources that are in a direction away from the desired direction.
Both of these types of microphones have a fixed directionality which provides good noise reduction from a direction in back of the microphone. However, typical directional microphones suffer from a number of disadvantages such as poor noise reduction for noise sources in front of the microphone, less than impressive noise reduction performance in low frequency bands such as those of a speech signal (which typically are on the order of 300-500 Hz), and colorization problems created by the tight dependency of the microphone's directionality in frequency. Thus, the frequency response of the microphone at "off axis" angles becomes irregular and the output may sound odd.
Microphone arrays (typically comprising five or eleven elements which are acoustically summed using analog technology) may be used to provide a directional pick-up pattern similar to a shotgun microphone or parabolic dish. In these types of microphones, the directionality is fixed, and the frequency response is, by mathematical definition, limited to a range from 500-5,000 Hz. The only way to improve the performance of this type of microphone is to increase the physical size of the array or utilize more individual microphones in the array. Due to the frequency response limitation which interferes with and cuts off the reception of speech signals, shotgun or parabolic microphones typically are preferred.
Hand-held microphones may be used for interview purposes. An important criteria for this application is the rejection of unwanted background noise, especially when the interview is conducted outside where various noise sources may be present in addition to the desired target source. While shotgun or parabolic microphones allow background noise to be rejected, these devices are impractical for use in an interview situation due to their large size, awkward performance at close range, and difficulties associated with holding the device.
Digital technology offers a technique known as beamforming in which signals from an array of spatially distributed sensor elements are combined in a way to enhance the signals coming from a desired direction while suppressing signals coming from directions other than the desired direction. This has the capability of providing the same directionality as would be provided by an analog microphone with the same size as the sensor array. In general, there are two beamforming techniques which are discussed in greater detail hereafter.
First, a non-adaptive beamformer may include a filter having a number of predetermined coefficients which allows the beamformer to exhibit maximum sensitivity or minimum sensitivity (a null) along a desired direction. The performance of a non-adaptive beamformer is limited because the predetermined filter coefficients do not allow nulls to be placed in the direction of interferences that may exist or to be moved about in a dynamically changing environment. Second, an adaptive beamformer includes a filter having coefficients that are continuously updated to allow the beamformer to adapt to the changing location of a desired signal in a dynamically changing environment. Thus, adaptive beamformers allow nulls to be placed in accordance with the movement of noise sources in a changing environment.
While adaptive beamformers provide significant advantages over a comparable analog device, adaptive beamforming devices are limited in resolution, dynamic range, and signal to noise ratio and are difficult to incorporate in and utilize with a directional microphone such as a shotgun microphone.